The stone he put in its place looked exactly like Jon's stone, except that it wasn't. It was the Similarity Target Stone. It worked just like the normal stone, except that the wishes would always be granted to one specific group of people, as if each of them made the wish themselves. If those people didn't hear the wish, then they wouldn't notice anything different, just as how the normal stone worked.
The stone always targeted a specific group of people, like all the jocks, or all the cheerleaders, or all the Goths or geeks. It was possible that the stone could target other "groups" like all the teachers or all the police officers or all the parents. But whichever "group" it was, the stone would always grant wishes to it, and only it.
But which "group" was the target of this stone?